Method for separating ashes in combustion installations

ABSTRACT

The invention relates to a method for separating ashes from the exhaust gases of combustion installations by means of separating devices. According to the invention, a hydrophobic, pyrogenically produced silicic acid is introduced into the flow of exhaust gas.

The invention relates to a process for separating ashes in combustioninstallations.

Combustion installations such as coal-fired power stations and inparticular waste incinerators are required to separate the ashes arisingon combustion from the waste gases and to dispose of them in a hazardouswaste landfill site or put them to another approved use.

It is known to separate the ashes from waste gases by using filters orfilter systems which are connected in series. One filter system used toseparate ultra-fine ashes is an electrostatic dust filter.

In the electrostatic dust filter, the ultra-fine ash, which cannot beseparated on the upstream surfaces, is ionised with high voltages. Thecharged particles then migrate to the oppositely charged separatorplate, from where they are pushed into a hopper by a scraper.

Under this hopper are located conveying means to a bunker, from whichthe residues are transported onwards to a landfill site, for example bytruck.

The known process has the disadvantage that the very finely divided dustbuilds up in the filter and clogs the hopper, preventing the dust fromtrickling down onto the conveying means.

The object of the invention was accordingly to provide a process forseparating ashes in combustion installations which does not exhibit saiddisadvantage.

The invention provides a process for purifying waste gases fromcombustion installations by means of separation apparatuses, whichprocess is characterised in that a hydrophobised, pyrogenically producedsilica is introduced into the waste gas stream, said silica beingvortexed with the ash particles.

In a preferred embodiment of the invention, the hydrophobic,pyrogenically produced silica may be added upstream from the separationapparatus, such as for example the electrostatic dust filter.

The hydrophobised, pyrogenically produced silica used may comprisesilicas which have been surface-modified or hydrophobised with thefollowing substances: dimethyldichlorosilane.

The hydrophobic, pyrogenically produced silica may be introduced, forexample, by means of blowing.

The hydrophobic, pyrogenically produced silica is known from Ullmann'sEnzyklopadie der technischen Chemie, 4th edition, volume 21, pages 466to 467.

The hydrophobic, pyrogenically produced silica may be added in aquantity of 0.1 to 0.2 kg per tonne of incinerated domestic waste.

The commercially available grades of silica (hydrophobic Aerosil®)listed in Table 1 may be used as the hydrophobic, pyrogenically producedsilica. TABLE 1 Hydrophobic AEROSIL ® AEROSIL AEROSIL AEROSIL AEROSILAEROSIL AEROSIL AEROSIL AEROSIL AEROSIL AEROSIL Test method R972 R974R202 R805 R812 R812S R104 R106 R8200 R816 Behaviour hydrophobic towardswater Appearance loose white powder BET surface 110 ± 20 170 ± 20 100 ±20 150 ± 25 260 ± 30 220 ± 25 150 ± 25 150 ± 30 160 ± 25 170 ± 25 area¹⁾m²/g Average primary 16 12 14 12 7 7 12 7 12 12 particle size nm Tampeddensity, 50 50 50 50 50 50 50 50 140 40 approx. value²⁾ standard productg/l compacted 90 90 90 product (suffix “V”) g/l Drying loss³⁾ <0.5 <0.5<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <1.0 (2 h at 105° C.) on departurefrom the supplier's works % Ignition loss⁴⁾⁷⁾ <2 <2 4-6 5-7 1.0-2.51.3-3.0 1.0-2.5 1.0-2.5 2.5-3.5 2.4-4.0 (2 h at 1000° C.) % C content %0.6-1.2 0.7-1.3 3.5-5.0 4.5-6.5 2.0-3.0 3.0-4.0 1-2 1.5-3.0 2.0-4.01.2-2.2 pH value⁵⁾¹⁰⁾ % 3.6-4.4 3.7-4.7 4-6 3.5-5.5 5.5-7.55.5-7.5 >4.0 >3.7 >5.0 4.4-5.5 SiO₂ ⁸⁾% >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 >99.8 Al₂O₃ ⁸⁾ %<0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 Fe₂O₃ ⁸⁾ % <0.01<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 TiO₂ ⁸⁾ % 0.030.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 HCl¹¹⁾ % <0.05 <0.1 <0.025<0.025 <0.025 <0.025 <0.02 <0.025 <0.025 <0.025¹⁾on the basis of DIN 66131²⁾on the basis of DIN ISO 4787/XI, JIS K 51018/18 (unscreened)³⁾on the basis of DIN ISO 787/II ASTM D 280, JIS K 5101/21⁴⁾on the basis of DIN 55921, ASTM D 1208, JIS K 5101/23⁵⁾on the basis of DIN ISO 787/IX, ASTM D 1208, JIS K 5101/24⁷⁾relative to material dried for 2 hours at 105° C.⁸⁾relative to material calcined for 2 hours at 1000° C.¹⁰⁾in water:methanol = 1:1¹¹⁾HCl content is part of ignition loss

In a preferred embodiment of the invention, the hydrophobic,pyrogenically produced silica Aerosil 972 may be used.

Silica Aerosil R 972 exhibits the following physicochemical parameters:Test method Aerosil R 972 Behaviour towards water hydrophobic Appearanceloose white powder BET surface area¹⁾ m²/g 110 ± 20 Average primaryparticle size nm 16 Tamped density/approx. value²⁾ standard product g/l50 compacted product (suffix “V”) g/l 90 Drying loss³⁾ % <0.5 (2 hoursat 105° C.) on departure from the supplier's works Ignition loss⁴⁾⁷⁾ %<2 (2 hours 1000° C.) C content % 0.6-1.2 pH value⁵⁾¹⁰⁾ 3.6-4.4 SiO₂ ⁸⁾% >99.8 Al₂O₃ ⁸⁾ % >0.05 Fe₂O₃ ⁸⁾ % >0.01 TiO₂ ⁸⁾ % >0.03 HCl⁸⁾¹¹⁾% >0.05 Drum size (net) kg 10¹⁾on the basis of DIN 66131²⁾on the basis of DIN ISO 787/XI, JIS K 5101/18 (unscreened)³⁾on the basis of DIN ISO 787/II, ASTM D 280, JIS K 5101/21⁴⁾on the basis of DIN 55921, ASTM D 1208, JIS K 5101/23⁵⁾on the basis of DIN ISO 787/IX, ASTM D 1208, JIS K 5101/23⁷⁾relative to material dried for 2 hours at 105° C.⁸⁾relative to material calcined for 2 hours at 1000° C.¹⁰⁾in water:methanol = 1:1¹¹⁾HCl content is part of ignition loss

The process according to the invention has the advantage that theultra-fine ash no longer builds up in the hopper and, as a consequence,the hopper also no longer becomes clogged.

The process according to the invention has been successfully trialledunder practical conditions in collaboration with Mr. Wolfgang Zieger andMr. Franz W. Albert at the Mannheim combined heat and powerstation/refuse incinerator.

The process according to the invention is illustrated and described ingreater detail with reference the drawings, which relate to theschematic diagram of the Mannheim refuse incinerator:

FIG. 1: possible addition points for adding Aerosil R 972 in thevicinity of the spray dryer 2(3) and the electrostatic dust filter 3(4)

FIG. 2: a possible addition point for adding Aerosil R972 in thevicinity of the gas inlet of the electrostatic dust filter downstreamfrom the end of the boiler and upstream from the electrostatic dustfilter

FIG. 3: a possible addition point for adding Aerosil R972 downstreamfrom the end of the boiler and upstream from the woven fabric filter

According to FIG. 1, Aerosil R 972 is added at various points in theflue gas removal zone. The Aerosil R 972 may be added to the productsuspension in the mixing apparatus 14. It may be introduced via thecompressed air for the spray dryer.

It may be introduced at the outlet from the spray dryer. According toFIG. 2 and FIG. 3, addition is made in the boiler zone at the gas inletupstream from the electrostatic dust filter or the woven fabric filterrespectively.

1. A process for purifying waste gases from combustion installations bymeans of separation apparatuses comprising introducing a hydrophobic,pyrogenically produced silica into the waste gas stream so that thesilica mixes with the ash particles contained in the waste gas streamassociated with the separation apparatus.
 2. A process according toclaim 1, further comprising introducing hydrophobic, pyrogenicallyproduced silica upstream from the separation apparatus.